Such an MAS NMR probe head arrangement is known from U.S. Pat. No. 7,915,893 B2.
Nuclear magnetic resonance (NMR) spectroscopy is a powerful analysis method, with which the chemical composition of measurement substances (samples) can be investigated. The measurement substance is exposed to a strong, static magnetic field, which leads to the alignment of nuclear spins in the measurement substance. After exposure to radio frequency (RF) pulses, RF signals emitted by the measurement substance are detected and evaluated to determine the chemical composition of the measurement substance.
NMR spectroscopy of solid samples shows significant line broadening in measured NMR spectra due to anisotropic interactions between nuclei in the sample. For solid-state samples, the known practice is to rotate the measurement substance during the NMR measurement at the so-called “magic angle” of about 54.7° with respect to the direction of the static magnetic field (MAS: “magic angle spinning”). This makes it possible to reduce a line broadening due to dipole interactions, quadrupole interactions, and the anisotropic portion of the chemical shift—and/or to extinguish them given sufficiently high rotational frequencies.
For this purpose, the measurement substance is typically filled into an essentially cylindrical sample tube, called a rotor, and placed in a stator. The rotor is rotated relative to the stator, with the rotor floating in the stator; suitable gas flows are used for this purpose. There is an RF coil arrangement which surrounds the stator to emit RF pulses into the measurement substance and/or to receive RF signals from the measurement substance.
A stator which is constructed with a housing tube and an inner sleeve is known from U.S. Pat. No. 7,915,893 B2. A rotor is arranged in the inner sleeve. Nozzle outlet openings which point toward the rotor, for two bearings, are constructed in the inner sleeve at the same radii. One of the bearings cooperates with a cylindrical section of the rotor, and the other bearing cooperates with a conical end portion of the rotor. The stator is surrounded by a solenoid coil and a ceramic coil body, wherein the stator extends through the ceramic coil body. The stator is glued or soldered at both ends with distributor rings which have the same diameter as the ceramic coil body. Due to the fixed connection of the stator to the distributor rings, the stator is permanently arranged in the ceramic coil body. Therefore, in the event of a defect in the stator, it can only be replaced together with the ceramic coil body and the solenoid coil. This is a time-consuming and expensive process. A further design with a permanently installed stator is known from U.S. Pat. No. 7,282,919 B2.
US 2016/0334478 A1 discloses an MAS NMR probe head with a rotor and stator, having two pneumatic radial bearings and a bottom bearing. An RF coil arrangement can be arranged between the parts of the radial bearings used for the stator. The rotor is cylindrical, with equal radii on both radial bearings. Likewise, the nozzle outlet openings in the stator have the same radius on both radial bearings. In this design, the probe head can be partially disassembled in case of a defect in the stator, and a stator part can be removed radially. However, this is very complicated. If the RF coil arrangement is arranged between the parts of the radial bearing used for the stator, it is not possible to insert or extract the stator in the axial direction. Similar arrangements are known from DE 10 2013 201 110 B3 and U.S. Pat. No. 7,196,521 B2.
From U.S. Pat. No. 8,212,565 B2 and U.S. Pat. No. 4,511,841 it has become known to insert an RF coil in the radial direction in an opening in the stator. In U.S. Pat. No. 6,320,384 B1 and U.S. Pat. No. 6,803,764 B2, the RF coil is inserted into a stator structure and surrounded by it. Two-part stators, which are assembled from both sides, are known from U.S. Pat. No. 9,335,389 and US 2010/0 321 018 A1.